This document discusses various methods for synthesizing silver nanoparticles (Ag NPs). It covers physical approaches like evaporation-condensation and laser ablation. It then discusses several chemical approaches for reduction of silver ions to produce Ag NPs, including reduction by tri-sodium citrate, sodium borohydride, UV irradiation, gamma irradiation, laser irradiation, microwave irradiation, sonochemical reduction, and electrochemical methods. It notes the advantages of these chemical synthesis techniques and how stabilizing agents prevent agglomeration of the nanoparticles.

1. Synthesis of silver nanoparticles
By
Ahmed Mostafa Hussein
Assistant Lecturer, Dental Biomaterials Department
Faculty of Dentistry, Mansoura University
2016
1

2. 2
Silver nanoparticles

3. Items to be covered
Introduction
Synthesis of Silver nanoparticles
* Physical approaches
* Chemical approaches
* Biological approaches
Applications of Silver nanoparticles
Mechanisms of antibacterial effect
Toxicity of silver nanoparticles
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4. Introduction
 Recently, nanoparticle (NP) synthesis is among the
most interesting scientific fields.
 Broadly speaking, there are two approaches to
nanoparticle production:
* Top-down: milling generates small particles from the
corresponding bulk materials
* Bottom-up: produces nanoparticles by starting from
the atomic level
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5.  There is growing attention to produce nanoparticles
(NPs) using environmental friendly methods (green
chemistry).
 Green synthesis approaches include polysaccharides,
biological and irradiation methods which have
advantages over conventional methods involving
chemical agents associated with environmental
toxicity.
5

6.  Various physical, chemical and biological synthetic
methods have been developed to obtain silver
nanoparticles (Ag NPs) of various shapes and sizes.
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7. Methods for synthesis of silver nanoparticles
A. Physical approaches
1) Evaporation-condensation
2) Laser ablation
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8. B. Chemical approaches
1) Reduction by tri-sodium citrate
2) Reduction by sodium borohydride
3) UV irradiation
4) Gamma irradiation
5) Laser irradiation
6) Microwave irradiation
7) Sonochemical reduction
8) Sonoelectrochemical method
9) Electrochemical method
10) Polysaccharide method
11) Tollens method
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9. C. Biological approaches
1) Synthesis of Ag NPs by bacteria
2) Synthesis of Ag NPs by fungi
3) Synthesis of Ag NPs by plants
9

10. A) Physical approaches
 The most important physical approaches include evaporation-
condensation and laser ablation.
1) Evaporation-condensation
 Vaporize the material into gas, and then cool the gas.
a) Using a tube furnace has some disadvantages
 The tube furnace consumes a great amount of energy.
 Raises the environmental temperature around the source
material.
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11.  Requires power consumption of more than several kilowatts .
 Requires a preheating time of several tens of minutes to reach a
stable operating temperature.
b) Using a small ceramic heater with a local heating source
 The evaporated vapor can cool faster than tube furnace.
 This physical method can be used for:
1. Formation of small nanoparticles in high concentration.
2. Formation of nanoparticles for long-term experiments for
inhalation toxicity studies.
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12. B) Laser ablation
 Laser ablation of metallic bulk materials in solution.
 Laser ablation can vaporize materials that cannot readily be
evaporated.
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Laser ablation in
liquid medium

13. The produced Ag NPs depend upon:
1. Wavelength of the laser
2. The duration of the laser pulses (in the femto-, pico-
and nanosecond regime)
3. The ablation time duration
4. The liquid medium
5. The presence of surfactant
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14. Advantages of laser ablation technique compared to other
methods for production of metal colloids:
1. Absence of chemical reducing agents
2. Pure and uncontaminated metal colloids can be prepared
by this technique.
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15. Advantages of physical approaches in comparison with chemical
processes
1. Absence of solvent contamination in the prepared thin films
2. Uniformity of NPs distribution
15

16. B) Chemical approaches
 The most common approach for synthesis of silver
nanoparticles is chemical reduction.
 In general, different reducing agents such as;
Sodium citrate,
Ascorbate,
Sodium borohydride (NaBH4),
Elemental hydrogen,
Polyol process
Tollens reagent
are used for reduction of silver ions.
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17.  The reducing agents reduce silver ions (Ag+) and lead to the
formation of metallic silver (Ag0), which is followed by
agglomeration into clusters. These clusters eventually lead to
formation of metallic colloidal silver particles.
 It is important to use protective agents to stabilize dispersive
nanoparticles during the course of metal nanoparticle
preparation, and protect the nanoparticles, avoiding their
agglomeration.
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18.  The presence of surfactants comprising functionalities (e.g.
thiols, amines, acids and alcohols) for interactions with
particle surfaces can stabilize particle growth and protect
particles from sedimentation and/or agglomeration.
 Stabilizers of nanoparticles include surfactants and polymers
with different functional groups, such as –COOH and –NH2.
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19.  Polymeric compounds such as polyvinyl alcohol (PVA) and
polyvinyl pyrrolidone (PVP) have been reported to be effective
protective agents to stabilize nanoparticles.
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20. 20
Silver nanoparticles

21. 1) Reduction by tri-sodium citrate
Steps
1. Dissolve 0.09 g AgNO3 in 500 ml water
2. Heating near boiling (95–98°C)
3. Add 1% tri-sodium citrate solution (0.1 g dissolved in 10 ml
water) dropwise, one drop per second
4. Continue heating 95–98°C (near boiling) for 15–90 min.
Yellow color indicates the formation of Ag NPs.
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22. 5. Wait until reaching room temperature, and then store in the
dark at 2–8°C.
 Citrate ions simultaneously act as reducing and stabilizing
agent.
 The silver colloidal particles possess a negative charge due to
the adsorbed citrate ions; a repulsive force works along
particles and prevents aggregation.
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23.  At low and high citrate concentrations (5 x 10-5 and 1.5 x 10-3
mol litre-1, respectively), coarse and defective silver
aggregates are formed.
 At intermediate concentrations of the stabilizing agent [in the
range of (1-5) x 10-4 mol litre-1], spherical nanoparticles with a
sufficiently narrow size distribution (8-11 nm in diameter) are
obtained.
 Low citrate concentrations insufficient stabilization
aggregation.
 High citrate concentrations destabilization.
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24.  The maximum degree of reduction is reached:
– In 40 min for a molar ratio [Ag] : [cit] = 1 : 1
[c(AgNO3) = 1 mmol litre-1]
– In 15 min for [Ag] : [cit] = 1 : 5.
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25. 2) Reduction by sodium borohydride (NaBH4)
 One of the first publications on the preparation of silver NP by
the borohydride method described the reduction of AgNO3
solution cooled to 0°C with a sixfold excess of NaBH4 solution
on active stirring.
 Another method:
1) 0.00386 g of sodium borohydride (NaBH4) was diluted with
water to 50 mL in a volumetric flask.
2) Keep in an ice bath to prevent degradation.
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26. 3. Weighing out 0.00442 g AgNO3 of which is diluted with
water in a 25 ml volumetric flask.
4. Add 12 mL of the ice cold prepared aqueous solution of NaBH4
to 4 mL of the AgNO3 solution with vigorous stirring.
This resulted in a color change to light yellow.
5. Stirring was continued until the reaction reached room
temperature.
 This synthesis procedure routinely yields particles of narrow
size distribution.
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27.  The synthesis of Ag NPs stabilized by PVA was reported, which
involved the reduction of AgNO3 in an aqueous solution with a
1.2-fold excess of NaBH4 in the presence of PVA.
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28. 3) UV irradiation
 UV irradiation can be used for synthesis of Ag NPs in the
presence of citrate, PVP or PVA.
4) Gamma irradiation (γ-irradiation)
 Chitosan, as a stabilizer, can be used in the preparation of Ag
NPs by gamma irradiation.
 Ag NPs can be synthesized by γ-ray irradiation of acetic water
solutions containing AgNO3 and chitosan.
 Ag NPs can be produced by irradiating a solution, prepared by
mixing AgNO3 and PVA.
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29. In case of γ- and UV-irradiation
 Large number of hydrated electrons (e−
aq) and H• are produced
during radiolysis of aqueous solutions by irradiation.
 They are strong reducing agents. Therefore, they can reduce
metal ions into zero-valent metal particles.
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30.  An OH• radical scavenger, such as primary or secondary
alcohols or formate ions, is added into the precursor solutions
before irradiation.
 For example, isopropanol can scavenge OH• and H• radicals
and at the same time changes into the secondary radicals,
which eventually reduce metal ions (M+) into zero-valent
atoms (M0).
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31. 5) Laser irradiation
 Laser irradiation (Nd:YAG 500nm) of an aqueous solution of
silver salt and surfactant can produce Ag NPs with a well-
defined shape and size distribution.
6) Microwave irradiation
 Microwaves in combination with polyol process were applied
in the synthesis of Ag NPs using ethylene glycol and PVP as
reducing and stabilizing agents, respectively.
31

32.  In a typical polyol process, inorganic salt is reduced by the
polyol (e.g. ethylene glycol which serves as both a solvent and
a reducing agent) at a high temperature.
 Large-scale and size-controlled Ag NPs could be rapidly
synthesized under microwave irradiation from an aqueous
solution of AgNO3 and tri-sodium citrate in the presence of
formaldehyde as a reducing agent.
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33. 7) Sonochemical reduction
 It involves the usage of ultrasonic waves to induce cavitations,
a phenomenon whereby the passage of ultrasonic waves
through an aqueous solution yields microscopic bubbles that
expand and ultimately burst.
 Sonolysis of water accounts for these sonochemical
reductions; more specifically, sonochemically generated H•
radicals are considered to act as reducing agents.
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34.  Often, organic additives (e.g. 2-propanol or surfactants) are
added to produce a secondary radical species, which can
significantly promote the reduction rate.
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35. 8) Sonoelectrochemical method
 It utilizes ultrasonic power primarily to manipulate the
material mechanically.
 The pulsed sonoelectrochemical synthetic method involves
alternating sonic and electric pulses.
 The electrolyte composition plays a crucial role in shape
formation.
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36. 9) Electrochemical method
 It is possible to control particle size by adjusting the
electrolysis parameters and to improve homogeneity of Ag
NPs by changing the composition of the electrolytic solution.
 PVP can be used to protect Ag NPs from agglomeration,
significantly reduces silver deposition rate and promotes silver
nucleation & Ag NPs formation rate.
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37. Electrochemical method for synthesis of Ag NPs
37

38.  The rate of reaction was found to increase with:
– Decrease in the distance between the electrodes (1–2 cm)
– Increase in the voltage (5–50 V DC)
– Increase in the temperature
 A longer reaction time resulted in:
– Larger size of Ag NPs
– Higher concentration of Ag NPs
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39.  Alternatively, the cathode could be other metals such as
platinum.
 The presence of PVA (1–100 ppm):
– Acts as supporting electrolyte
– Accelerates the NP nucleation and growth
– Produces highly concentrated suspensions of NPs
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40. 40
Electrochemical method for synthesis of Ag NPs

41. 10) Polysaccharide method
 Ag NPs can be prepared using water as an environmental
friendly solvent and polysaccharides as reducing/capping
agents.
 For instance, the synthesis of starch-Ag NPs can be carried out
with starch (as a capping agent) and glucose (as a reducing
agent).
41

42.  Also, Ag NPs can be synthesized by mixing two solutions of
AgNO3 containing starch (as a capping agent) and NaOH
solutions containing glucose (as a reducing agent).
42

43. 11) Tollens method
 It involves the reduction of [Ag(NH3)2]+ + (as a Tollens reagent)
by an aldehyde.
 In the modified Tollens procedure, silver ions are reduced by
saccharides in the presence of ammonia.
[Ag(NH3)2]+(aq) + RCHO(aq) Ag(s)+RCOOH(aq)
where RCHO is an aldehyde or a carbohydrate.
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44.  Ag NPs with controllable sizes can be synthesized by reduction
of [Ag(NH3)2]+ with glucose, galactose, maltose and lactose.
The NP synthesis was carried out at various ammonia
concentrations.
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45. 45
Silver nanoparticles

46. 46
Colors of silver nanoparticles:
yellow, orange & brown

47. C) Biological approaches
 The bioreduction of metal ions by combinations of
biomolecules found in the extracts of certain organisms (e.g.,
enzymes/proteins, amino acids, polysaccharides and vitamins)
is environmentally benign.
47

48. 1) Synthesis of Ag NPs by bacteria
 Culture supernatants of bacteria can be used for synthesis of
Ag NPs.
 Such as culture supernatants of E. coli, Klebsiella pneumonia,
B. subtilis, Enterobacter cloacae and Bacillus licheniformis.
 Also, Lactobacillus strains can be used for synthesis of Ag NPs.
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49. 2) Synthesis of Ag NPs by fungi
 Such as Fusarium oxysporum, Phanerochaete chrysosporium,
Aspergillus flavus and Aspergillus fumigatus.
 The stability of Ag NPs is due to capping by proteins.
 Proteins, enzymes, organic acids and polysaccharides are
responsible for the formation of Ag NPs.
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50. 3) Synthesis of Ag NPs by plants
Advantages
1) Green synthesis & eco-friendly
2) Low cost
3) Can be used for large scale synthesis
4) No need to use high pressure, energy, temperature and
toxic chemicals
5) Not require any special culture preparation and isolation
techniques
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51.  Such as green tea extract, black tea leaf extract, Polyphenols,
flavonoids, alfalfa, geranium, Brassica juncea, Datura metel
leaf extract, Pinus desiflora, Diospyros kaki, Ginko biloba,
Magnolia kobus, Platanus orientalis leaf broths, Nelumbo
nucifera, Sorbus aucuparia leaf extract, Euphorbia hirta leaf
extract, Eucalyptus citriodora, Ficus bengalensis, Garcinia
mangostana leaf extract, Ocimum sanctum leaf extract,
Cacumen platycladi extract and Cinnamon zeylanicum bark
extract.
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52. * Green tea extract can be used as reducing and stabilizing agent
for the biosynthesis of Ag NPs in an aqueous solution in
ambient conditions.
* Plants (especially plant extracts) are able to reduce silver ions
faster than fungi or bacteria.
* In biological synthetic methods, it was shown that the Ag NPs
produced by plants are more stable in comparison with those
produced by other organisms.
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53. Applications of silver nanoparticles
Introduction
 The use of silver in wound management can be traced back to
the 18th century, during which AgNO3 was used in the
treatment of ulcers.
 The antimicrobial activity of the silver ions was first identified
in the 19th century.
 Colloidal silver was accepted by the US Food and Drug
Administration (FDA) as being effective for wound
management in the 1920s.
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54.  However, after the introduction of penicillin in the 1940s,
antibiotics became the standard treatment for bacterial
infections and the use of silver diminished.
 Silver began to be used again for the management of burn
patients in the 1960s, this time in the form of 0.5% AgNO3
solution.
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55.  AgNO3 was combined with a sulphonamide antibiotic in 1968
to produce silver sulfadiazine cream, which created a broader
spectrum silver-based antibacterial that continued to be
prescribed mostly for the management of burns.
 More recently, clinicians have turned to wound dressings that
incorporate varying levels of silver, because the emergence
and increase of antibiotic-resistant bacteria have resulted in
clinical limitations in the prescription of antibiotics.
55

56.  Ag NPs are of great interest due to their extremely small size
and large surface to volume ratio, which lead to differences in
their properties compared to their bulk counterparts.
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57. Medical and dental applications
1) Bone cement
2) Implantable devices
3) Additive in polymerizable dental materials
4) Toothpastes
5) Surgical gowns
6) Face masks
7) Wound dressing and burn treatments
8) Coating plastic catheters
9) Coating of endotracheal tube
10) Disinfecting medical devices
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58. Nasal spray
58
Toothpaste

59. Disinfectants
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60. Other applications
1) Food storage packaging
2) Textile coatings, socks and athletic clothing
3) Packaging
4) Cosmetics
5) Water treatment
6) Washing machines
7) Detergents, soaps and shampoos
8) Air and water filters
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61. Socks
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62. Mechanisms of antibacterial effect
 The bactericidal action of NPs increases as the particle size
decreases.
 Ag NPs with diameters smaller than 10 nm can directly
interact with a bacterium.
Ag NPs
 Ag NPs increase the permeability of membrane.
 Ag NPs bind to thiol groups (–SH) in the respiratory enzymes
and deactivate the enzymes.
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63.  Ag NPs interact with respiratory enzymes and generate
reactive oxygen species Oxidative stress apoptosis
 Ag NPs interact with DNA (phosphorus-containing compound)
and inhibit its replication.
 Ag NPs undergo slow oxidation and form Ag+.
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64. Ag+
 Ag+ destructs peptidoglycan layer in the cell wall.
 Ag+ causes structural change in the bacterial cell wall &
nuclear membrane.
 Ag+ affects ribosomes (binds to 30S ribosomal subunit) and
inhibits protein synthesis.
 Ag+ interacts with thiol-containing proteins & enzymes.
 The most antibacterial to least: triangular, spherical, and then
rod-shaped NPs.
 The triangular NPs have more active facets than spherical NPs.
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65.  There is a positive synergistic effect of Ag NPs and different
antibiotics against S. aureus and E. coli.
 Ag NPs are the powerful weapons against the multidrug-
resistant bacteria such as ampicillin-resistant Escherichia coli,
erythromycin-resistant Streptococcus pyogenes, methicillin-
resistant Staphylococcus aureus and vancomycin-resistant
Staphylococcus aureus.
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66.  Ag NPs are more antimicrobial than copper, titanium,
magnesium, zinc and gold NPs.
 Note: Ag NPs inhibit HIV-1 virus replication.
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67. Toxicity of silver nanoparticles
 Argyria is the most frequent adverse outcome from exposure
to Ag NPs.
 For instance, prolonged ingestion of colloidal silver can
change the color of skin and cause blue-grey appearance of
the face (the symptoms of argyria).
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68. Argyria
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69. 69
Argyria

70.  Some authors have claimed that Ag NPs possess low or zero
toxicity to human cells.
 However, other studies show that Ag NPs are toxic to
mammalian cells derived from skin, liver, lung and brain.
 Generally, the toxicity of Ag NPs depends on their size, shape,
chemical composition and surface modification.
 Uncoated Ag NPs are more toxic than coated Ag NPs.
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71. Notes
 The most commonly used concentration of commercial Ag
NPs is 0.02 mg/ml = 20 ppm
ppm = μg/ml = mg/L = mg/1000ml
 Aldrich offers several Ag NPs suspended in a dilute aqueous
citrate buffer which weakly associates with the NP surface.
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72.  This citrate-based agent was selected, because the weakly
bound capping agent provides long term stability and is
readily displaced by various other molecules including thiols,
amines, polymers and proteins.
 Ag NPs should be stored at 4°C (2-8°C) in the dark, because
they are photosensitive.
 Ag NPs non-stabilized in a proper way undergo fast
oxidation and easily aggregate in solutions.
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73. 73
UV spectroscopy of silver nanoparticles

74.  The solution color and absorption spectra give an approximate
idea of the particle size.
 Nanoparticles were denoted as colloids until the invention of
nanotechnology.
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75. 75
PELCO NanoXact and BioPure
Silver Colloids/Nanoparticles

76. Thank you
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